Dynamics of molecular adsorbates exposed to ultrafast thermal gradients

Energy transfer concerned with how thermal energy is transported at molecular-length scales is a relatively new area and the physics governing this process are not yet well established. The experiments presented here represent initial steps toward developing a better understanding how thermal energy travels across solid-molecule interfaces as well as how thermal energy flows through molecules. The experimental scheme here is as follows. A molecular monolayer is bound on one end to a gold film, acting as an ultrafast heat bath. Using a laser flash-heating technique, the heat bath can be jumped to several hundred degrees Celsius in the duration of a few picoseconds. As the energy flows from the thermal reservoir into the molecules com- posing the monolayer, a coherent non-linear vibrational spectroscopy probes specific localized vibrational modes within the monolayer to monitor the presence of thermal disorder on the picosecond time scale. \n\nBy measuring the transit time of energy flowing through alkane-based monolayers of increasing length, the flow of thermal energy was found to be ballistic – or constant velocity – in nature as it traveled through the molecules. This data also strongly suggest that energy is deposited directly into a delocalized region of the alkane chains ∼0.8-nm in length. \n\nBenzylthiolate-based monolayers allowed the energy transfer dynamics for systems with sub-nanometer lengths. In addition to phonon heating, these monolayers experience electron-heating effects that excited high-energy vibrational mode through inelastic scattering events. For monolayers containing a phenyl group, the electron scattering range was found to be limited to ≤1 nm.